For many devices, particularly electrically energized devices such as electronic systems for communication, or data handling, or navigation control, for instance, it is very desirable to have some means of determining the cumulative startups. This is especially desirable when high reliability is required. The cumulative operations measurement provides a basis for determining when the device or apparatus should be given routine maintenance, or overhaul, or when a complete replacement should be made in order to assure reliability. Such reliability problems are especially important, for instance, in aircraft.
Prior cumulative operations measurement systems which measure operating time (referred to as "elapsed time indicators") have frequently employed electromechanical counter using synchronous motors or pulse circuits which actuate mechanical number registers which are similar to the mechanical registers in automobile odometers. These prior structures have a number of serious limitations and disadvantages. For instance, the accuracy of such devices is seriously limited by various factors. One of the most important factors is slow response to the initiation of the operating voltage resulting in a loss of up to five seconds for each startup of the apparatus being timed. These prior devices are also rather bulky in size. In order to keep the bulk down, the resolution is generally limited to about five decimal digits. This further reduces the precision with which the cumulative operating time is stored. Such electromechanical cumulative operating time indicators are also relatively costly, and each one adds a considerable weight. These factors are important when it is appreciated that a large number of such indicators may be required in a vehicle such as a aircraft where weight saving is vital.
Another problem with the electromechanical elapsed time indicators is that generally the only means of recording the cumulative operating time for the purpose of comparing prior readings, and for record-keeping purposes is to have the human operator take a visual reading of the elapsed time indicator and record that reading manually. This may typically involve removing a cover of the apparatus in which the elapsed time indicator is installed, and it is generally a laborious and time consuming activity. It would be much more convenient to have an elapsed time indicator which could be remotely read, and especially one which could be remotely read by a signal processor which could provide a permanent record automatically.
Still another problem with the electromechanical elapsed time indicators is that because of the different voltages and frequencies encountered in the machines for which cumulative operating time is to be recorded, different models of the time indicators must be provided which are adapted for operating at different voltages and frequencies. Furthermore, becuse of the limited resolution, different models must be provided an selected depending on the total anticipated operating time which is to be recorded.
Some of the above-mentioned disadvantages can be avoided by employing a coulometric elapsed time indicator in which a dc current causes the movement by plating of mercury molecules through an electrolyte gap in a column of mercury from one side to the other of the gap. The position of the meniscus of electrolyte is then a measure of time during which the plating current has persisted. Such devices are available from Curtis Instruments, Inc. of Mt. Kisco, N.Y., the present assignee of the present patent application. However, the coulometric elapsed time indicators also have some serious disadvantages including limited measurement resolution, undesirable changes in characteristics in response to changes in operating temperatures, and inherent physical limits of shock and vibration resistance.
Accordingly, it is an object of the invention to overcome one or more, and preferably all of the limitations and disadvantages of the prior elapsed time indicators as discussed immediately above.
Prior cumulative operations measurement systems which measure a count of the number of times operation is initiated (often referred to as "event counters") have frequently employed technology similar to that used in elapsed time indicators, such as pulse circuits which actuate mechanical number registers similar to the mechanical registers in automobile odometers. These prior structures have a number of the same limitations and disadvantages enumerated above for elapsed time indicators. These disadvantages include bulky size, limited numerical resolution, high cost, and considerable weight. The disadvantages also include the cumbersome procedures for recording the event count, and the requirement that different voltages and frequencies of signals to be counted and recorded must be dealt with.
Accordingly, it is another object of the invention to overcome one or more, and preferably all, of the limitations and disadvantages of prior even counters as discussed immediately above.
In accordance with the present invention, na operations measurement system is employed which makes exclusive use of solid state digital circuitry for accumulating an storing the accumulated operations in terms of digital counts. However, such a system can be very vulnerable to electrical "noise" which may be derived from the machine or apparatus for which operations are being accumulated, particularly since such a machine or apparatus may often include an electronic circuit or other components which give rise to oscillations or voltage spikes during startup.
Accordingly, it is another object of the invention to provide an operations measurement system which is remarkably free from the risk of false operation due to electrical noise on the input connections to the operations measurement system.
Further objects and advantages of the invention will be apparent from the following description and the accompanying drawings.